A hot cell installation for the handling of highly radioactive material may comprise a dozen or more interconnected high density concrete vaults, the concrete vault walls having a thickness of approximately three feet. Typically, hot cells are constructed in rows so as to share as many shielding walls as possible. Each row has a lead door at each end through which materials can be introduced or removed. The individual vaults, referred to as cells, are separated from adjacent cells by approximately three foot thick shield walls, also of high density concrete, each such separating wall having a large penetration accommodating sliding lead shield doors which permit interconnection of the cells and movement of objects and equipment between cells. These doors allow materials to be moved from cell to cell by use of an inter-cell transfer cart.
The cells are also equipped with leaded glass windows which allow viewing of the internal master-slave manipulators. This arrangement permits the remote handling of radioactive materials inside the cells and also permits the remote movement of optical equipment for detailed viewing of areas in the entire cell interior. The manipulators also allow remote operation of various other pieces of examination, handling, and machining equipment inside the cell. A typical overall length of a row of cells might be 70 yards. Depth of a cell typically ranges between 12 and 16 feet.
The individual hot cells are used for the disassembly, examination, and testing (both nondestructive and destructive) of irradiated nuclear fuel and materials highly contaminated with radioactivity. A typical use of a hot cell might be for optical examination and chemical sampling of spent fuel elements. The hot cells provide the necessary radiation shielding for the personnel outside the hot cells so that they can safely handle these materials.
During the processing of radioactive materials inside a cell, various items must be placed into the cells. In a typical project, materials must be inserted into a cell on average twice in an eight hour shift.
In general, materials have been moved into a cell by two primary methods. In the first method, some hot cells are equipped with water canals under the floor connecting the outside with the inside for this purpose. Underwater carts and elevators accept materials and move them through the water canal to the inside of the cell. The water provides continuous shielding. A second primary mechanism is an opening with a width in the vicinity of one and a half feet equipped with a double lead door mechanism. However, in practice such doors can only be installed in the end cells.
Each of these approaches to allowing materials to enter a hot cell during operations suffered from significant limitations. The first method is of course limited to transferring materials which can be submerged in water without damage or chemical reaction. Water canals are also difficult and expensive to construct and maintain. Moreover, materials often must be moved to or from a cell not connected to a water canal, and there is thus a problem of moving materials from cell to cell after delivery.
The second method is usable in practice only on end cells because of the need for and placement of viewing windows. Thus materials destined for or coming from interior cells must be manipulated remotely to or from the end cell adjacent to the door. The end cells are often blocked from other cells by alpha particle control enclosures in such a way that materials cannot move back and forth between cells.
These methods of introducing and removing materials to and from individual hot cells also suffered from a common shortcoming, namely that they required intricate maneuvering of the material to be inserted and are thus time consuming and exposure prone. It is apparent that if materials could be placed directly in or removed directly from individual hot cells substantial savings in time for operations and personnel exposure to radiation could be achieved.
A secondary mechanism exists for placing certain objects into a cell. This method makes use of approximately 3 inch diameter openings in the concrete filled with removable lead plugs. This method of insertion, though fast, was very limited in that only objects with a profile small enough to pass through a three inch diameter cylinder could enter the hot cell in this way.
More recently, a typical hot cell has been constructed with 8 inch diameter holes through the exterior shielded walls in the vicinity of, and usually above, the viewing windows. Such holes are left in hot cell walls during construction to accommodate later installation of periscopes, remote manipulation equipment, and the like. When the holes are not in use for such equipment, they are filled with steel plugs of approximately the same shielding value as the wall.
It became evident that if the hot cell plugs could be removed and replaced conveniently significant savings in time and personnel exposure could be realized by using these 8 inch holes as entry ports. The high density concrete exterior walls of cells are typically 36 inches thick with a "tenth value" thickness--the thickness of a material that attenuates radiation by a factor of ten--of 10 inches. The attenuation factor of three feet of high density concrete is thus very close to 4000. To obtain a comparable attenuation factor, the steel used for doors and plugs must be about 15 inches in thickness. Fifteen inch cylindrical steel plugs with a diameter of eight inches weigh about two hundred pounds.
Experience has proven that it is possible to use these eight inch holes to introduce a wide variety of materials into individual cells by removing and replacing the eight inch steel plugs. This access method avoids most of the limitations associated with the previous approaches. However, the weight of the plugs has until the present dictated the use of heavy equipment to remove and replace the steel plugs. The only method known up to the present time for moving the plugs out of and back into the holes was by means of a fork lift. This method required four personnel: a hot cell technician, a radiological controls technician, a rigger, and an operator, to work approximately two hours to gain access to a hot cell through a plug. The current invention overcomes these and other limitations of the prior art.